1. Technical Field
The present invention is directed to an operational timer circuit for monitoring the amount of time during which a device is in use. More particularly, the present invention is directed to an operational timer circuit for monitoring the time during which the device is under load.
2. Background Information
Elapsed time indicating meters, meters recording the time during which a device is operating, are well known in the art.
In U.S. Pat. No. 3,321,489 issued to Mackey et al., herein incorporated by reference, an elapsed processing time meter is disclosed for use with an electronic digital computer to record the computer time utilized to perform actual computing operations for billing and preventive maintenance purposes. The time meter records the time during which a digital computer is performing computations, defined as the time during which the computer's memory unit is in operation. A pick-up device is located adjacent to the memory unit to pick up radiant electrical signals when the memory unit is in operation. The output from the pick-up device is amplified by a first amplifier and the output from the first amplifier is amplified by a second amplifier whose output is used to trigger a power relay. The output of the second amplifier is coupled to the coil of the power relay, and when this coil is energized, the power relay actuates a switch, allowing power to flow to a clock, thereby recording the time during which the computer's memory unit is in use. The circuitry also includes a power supply for generating the proper positive and negative voltages for the first and second amplifiers and the power relay. The power supply circuitry includes a fused power transformer, rectifying diode, and filtering resistors and capacitors.
Although adequate for its purpose, systems such as Mackey are deficient for several reasons. As more than one amplifier stage is needed, the circuitry is more complex and the additional components inherently reduce circuit reliability. Additionally the requirement of a separate power supply adds to a degradation of circuit reliability. Further, the requirement of a power transformer increases the space requirement of the circuit. The power relay further adds to overall circuit size, in addition to relatively large power requirements required by a power relay.
In U.S. Pat. No. 3,258,908 issued to Fischer, herein incorporated by reference, an elapsed time indicating meter is shown for use with tape cartridge recorders for reading out the total unconsumed time available for recording on a given tape. The circuit in Fischer is connected to the control output leads from a recorder, and the voltage therefrom is rectified and filtered via a clipper circuit, the output of which is coupled across the base and collector of a transistor for amplification. A coil is connected to the emitter of the transistor for activating a switch when the coil is energized. The switch turns the timer motor on, thereby tracking the remaining recording time. Also included in the tape cartridge timer circuit is a transformer, rectifier and filter arrangement for supplying power to the coil and amplifier. Although the Fischer circuit offers improved reliability from the above Mackey circuit by requiring only one amplifier stage, the problems associated with a power relay circuit and a power supply with a transformer are also inherent in Fischer.
Elapsed time indicating meters also find utility in recording the cumulative running time of electric ignition engines, examples of which are shown in U.S. Pat. No. 3,299,627 issued to Hart et al. and U.S. Pat. No. 3,948,039 issued to Leveraus, both of which are herein incorporated by reference.
The Hart circuit is connected to the positive and negative potentials of the battery ignition system, and the circuit is switchable by the engine's ignition switch. Connected across the battery terminals is an inductive coil in series with a transistor. The coil cooperates with a magnetic circuit to form the input for an electrical pulse counter comprising a pawl-ratchet mechanism. The pawl is connected to the armature of the coil and the ratchet is connected to a series of counting wheels by suitable gearing. Electrical pulses delivered to the coil index the ratchet and associated counting wheels at the desired rate. The electrical pulses are derived from a circuit both connected across the battery terminals and operatively coupled to the base of the transistor. The pulse delivery circuit includes an RC timer network wherein the voltage across the capacitor, upon reaching a predetermined threshold value, triggers a field-effect transistor, the output of which provides sufficient base current to the transistor in series with the coil to turn the transistor on, thereby energizing the coil and applying the requisite indexing pulse to the pulse counter.
The major design flow of circuits such as Hart et al. is that the circuit is not accident-proof. The ignition switch may be turned on accidentally and may remain on for several hours. A considerable lapse of time may transpire before it is noticed that the switch is on. Under these circumstances, it is rarely possible to know the actual running time of the vehicle since the time recorded on the meter is in error.
One possible solution to this problem is shown by Leveraus, wherein the timer circuit is operated by a signal which operates off of the tachometer. The Leveraus circuit employs a monolithic Darlington transistor in series with a solenoid to activate the time indicating meter. When the vehicle's tachometer is operating, a pulse signal from the alternator is both rectified and filtered and used to turn on a field-effect transistor, the output of which causes the Darlington transistor to turn on, thereby energizing the armature of the solenoid to send power to the time indicating meter.
All of the above circuits utilize a coil (solenoid) to switchably control the elapsed time indicating meter. As discussed above, the coils are both bulky and have a relatively large power requirement. Accordingly, although the cumulative time during which a device is operating is recorded, the load on the system is increased. This load not only shortens the life of the battery, but may further render the subsequent operation of the engine nonfunctional should the battery voltage drop below its requisite threshold. Furthermore, given the tendency towards decreasing the size of circuitry, the volume required by the solenoid and/or the power transformer may be unacceptable in many circumstances.
In addition to the deficiencies in the prior art relating to circuit size, cost, power requirements and reliability, as discussed above, the prior art timer circuits are also deficient in that they measure only the time a device is operating generally (merely turned on), as opposed to measuring the time during which a device is operating under load. The latter requirement finds particular utility in applications where periodic preventive maintenance is determined based upon load time usage, or where the load time usage is indicative of actual hours performing a service, such as vacuuming a carpet by maintenance personnel.